Pick up a copy of ATV Action, or dirtwheels, etc., Look in the back section.
Honestly...there's probably a dozen places in SoCal that do work for shees.
With year round riding, Glamis sand dunes, etc., out there in your neck of the woods, I'm sure you can find plenty of good, reputable engine builders.
Ask to talk to them, stop by...talk to a few of their customers....